This invention relates to a hydraulic master cylinder for use in a vehicle braking system.
In the hydraulic master cylinder hithertofore proposed, a housing constituting the main body of the master cylinder for receiving a piston slidably therein has been formed of material such as cast iron or cast aluminum alloy. When the housing is formed of cast iron, it is advantageous in that cast iron is corrosion-resistant with respect to braking fluid, but there are shortcomings in that the weight of the master cylinder increases, and the cost for machining a portion for fitting a braking lever thereto, a portion for fitting the master cylinder to the vehicle, and a portion constituting an oil reservoir, a portion constituting cylinder and oil passages or the like becomes expensive since these portions usually have complicated configurations and high accuracy is required in the machining operation.
When the housing is formed of cast aluminum alloy by utilizing a gravity casting process, it is possible to reduce the weight of the master cylinder, which can be easily cast and can be machined easily. However, the cast aluminum alloy has shortcomings in that excessive corrosion will be observed in the metal exposed to braking fluid under pressure and excessive wear will occur in the portion bearing the moving parts such as a piston, and internal defects in the cast material such as porosity or internal cavities will cause leakage of braking fluid or obstruction to smooth operation of the master cylinder.
When the housing is formed by a pressure die-casting process, it is possible to obtain a housing having a beautiful appearance and a complicated construction but usually the casting is porous and, thus, the housing must be protected from being exposed to braking fluid under pressure.